XIX.] EXPERIMENT. 427 



The electric arc is an invaluable means of exposing 

 metals or other conducting sulistances to the highest 

 known temperature. By its aid we learn not onl}'- that 

 all the metals can be vaporised, but that they all give off 

 distinctive rays of light. At the other extremity of the 

 scale, the intensely powerful freezing mixture devised by 

 Faraday, consisting of solid carbonic acid and ether mixed 

 in vacuo, enables us to observe the nature of substances at 

 temperatures immensely below any we meet with naturally 

 on the earth's surface. 



We can hardly realise now the importance of the in- 

 vention of the air-pump, previous to which invention it 

 was exceedingly difficult to experiment except under the 

 ordinary pressure of the atmosphere. The Torricellian 

 vacuum had been employed by the philosophers of the 

 Accademia del Cimento to show the behaviour of water, 

 smoke, sound, magnets, electric substances, &c., in vacuo, 

 but their experiments were often unsuccessful from the 

 difficulty of excluding air.^ 



Among the most constant circumstances under which 

 we live is the force of gravity, which does not vary, except 

 by a slight fraction of its amount, in any part of the earth's 

 crust or atmosphere to which we can attain. This force is 

 sufficient to overbear and disguise various actions, for in- 

 stance, the mutual gravitation of small bodies. It was an 

 interesting experiment of Plateau to neutralise the action 

 of gravity by placing substances in liquids of exactly the 

 same specific gravity. Thus a quantity of oil poured into 

 the middle of a suitable mixture of alcohol and water 

 assumes a s[)lierical shape ; on being made to rotate it 

 becomes splieroidal, and then successively separates into 

 a ring and a group of spherules. Thus we have an 

 illustration of the mode in which the planetary system 

 may have been produced,^ tliough the extreme difference 

 of scide prevents our arguing with confidence from the 

 experiment to the conditions of the nebular theory. 



It is possible that the so-called elements • are elementary 

 only to us, because we are restricted to temperatures at 

 which they are fixed. Lavoisier carefully defined an 



' Essayes of Natural Exjxrime.nU made in the Accademia del 

 Cimento. Eii;.,'lisli(;d by Kichard Waller, 1684, p. 40, &c. 

 - Plateau, Taylor's Scientijic Memoirs, vol. iv. pp. 16 — 43. 



